Depending upon whether the stem has threading for the clamp bolt, you can simply reverse the bolt and thread it on from the other side. Stick a penny or something in the slot between the halft and drive the bolt aginst this. This will force the sides apart and you should be able to slip your handlebars on thru.
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This is Africa, 1943. War spits out its violence overhead and the sandy graveyard swallows it up. Her name is King Nine, B-25, medium bomber, Twelfth Air Force. On a hot, still morning she took off from Tunisia to bomb the southern tip of Italy. An errant piece of flak tore a hole in a wing tank and, like a wounded bird, this is where she landed, not to return on this day, or any other day.